The objective of this project is to further develop and apply methods for demonstration with the electron microscope of sites of Ca binding or accumulation in order to explore the possibility that certain physiological and/or metabolic functions in neurons are regulated in coordination with membrane activity via Ca entry or release of Ca from internal stores. Neurons injected with Ca-buffers show altered morphology dependent upon the Ca-concentration of the injected solution. These changes are similar to those obtained by varying the extracellular Ca-concentration and treating the neurons with physiological stimuli that increase the cytoplasmic Ca-concentration. The freeze-fracture technique is being used to analyse the morphologic change in pigment granules of Aplysia neurons in response to light. Freeze-fracture studies of pigment granules of Aplysia neurons in light and dark are consistent with the interpretation of thin sections reported previously.